Schottky gate gallium nitride (GaN) transistors used in power switching applications suffer from undesirably high gate leakage current, which results in lower operational efficiency due to excessive power dissipation. Reverse gate leakage for a Schottky contact on a GaN device is typically 10 μA to 1 mA per millimeter (mm) of gate width depending on the construction of the device. For a Schottky gated GaN power transistor with 100 mm gate periphery, the gate-to-drain leakage would be on the order of 1 mA to 100 mA. In contrast, silicon-based power transistors having a dielectric deposited under the gate to provide an “insulated gate” have a much lower leakage current that ranges from about 1 nA to around 100 nA. Moreover, silicon has a natural advantage of an extremely high-quality native oxide, which is silicon dioxide (SiO2). As such, silicon metal oxide field effect transistors (Si MOSFETs) and insulated gate bipolar transistors (IGBTs) are relatively easily fabricated. Consequently, Si MOSFET devices and IGBT devices dominate the power transistor market. For many years, alternative semiconductors fabricated using wide bandgap semiconductors such as silicon carbide (SiC) and GaN have been evaluated in an attempt to achieve better performance than silicon devices. In most cases, a primary obstacle has been a lack of a high-quality insulator material.
Gallium nitride (GaN) metal oxide semiconductor—high electron mobility transistors (MOSHEMTs) using dielectrics such as SiO2, silicon nitride (SiN), hafnium dioxide (HfO2), aluminum oxide (Al2O3), and aluminum nitride (AlN) have been proposed, but the performance and reliability of these dielectrics have been limited by the poor interface properties of dielectrics on GaN. One problem is a large hysteresis in current-voltage (ID-VG) transfer characteristics after an application of drain voltage. Another problem is time-dependent gate oxide breakdown (TDDB) caused by relatively large reverse bias voltages applied during operation. Still other problems include threshold voltage instability and mobility degradation due to columbic scattering.
Compounding these problems is a need for an undesirably thick dielectric to support high electric fields that occur in high-voltage GaN transistors that operate under drain voltages that range between 600V and 1200V. For example, a typical high-voltage GaN MOSHEMT typically has an insulating gate layer with a thickness that ranges between 100 Angstroms (Å) and 500 Å. Further still, for a given density of trapped interface charge (Qit) a thick dielectric with a relatively low areal capacitance (Cox) results in a significant threshold voltage (Vth) shift (Δ) that is governed by the following mathematical relationship.ΔVth=Qit/Cox  EQ. 1
As illustrated by EQ. 1, a larger ΔVth occurs for a given Qit as Cox decreases as the insulating gate layer gets thicker. As a result, performance of a GaN MOSHEMT is degraded with increased gate layer thickness. At some point, the Vth is shifted too much to be practical. Thus, there exists a need for a low-leakage gate for GaN transistors that does not suffer from the disadvantages of insulated gate structures.